Cardiac arrest and ventricular fibrillation are life-threatening medical conditions that may be treated with external defibrillation. External defibrillation includes applying electrodes to the patient's chest and delivering an electric shock to the patient to depolarize the patient's heart and restore normal sinus rhythm. The chances that a patient's heart can be successfully defibrillated increase significantly if a defibrillation shock is applied quickly. In many cases, it is more expedient to bring a defibrillator to the patient than to bring the patient to a defibrillator.
Many external defibrillators are portable. Portable external defibrillators may be used in hospitals and outside hospital settings as well. Paramedics, emergency medical technicians and police officers, for example, may carry portable external defibrillators in their vehicles. In addition, automated external defibrillators (AED's) may be available in public venues such as airports, health clubs and auditoriums. Portable external defibrillators are compact and lightweight.
Before an external defibrillator is used to administer a shock, the energy to be delivered to the patient must be stored in an energy storage device, such as a capacitor. Many defibrillators use a charging circuit to transfer energy from a power source, such as an electrical outlet or a battery, to the energy storage device. When a switch is closed, the energy storage device delivers at least a part of the stored energy from electrode to electrode through the patient's chest. Delivery of energy from the energy storage device is completed in a few milliseconds.
Following administration of one shock, it may be necessary to administer another. The charging circuit draws energy from the power source and transfers the energy to the energy storage device. When the energy storage device is sufficiently charged, another shock may be administered. In order to charge the energy storage device quickly, the power source may be called upon to supply approximately fifty to one hundred fifty watts of power. The power supplied is a function of the voltage of the power source and the current supplied by the power source. To supply fifty to one hundred fifty watts, a ten-volt power source, for example, may be called upon to supply between five to fifteen amperes of current.
Some conventional external defibrillators are powered by a connection to an electrical outlet. These “line-powered” defibrillators carry risks of injury to the patient, to bystanders and to persons operating the defibrillator. In particular, a line-powered defibrillator carries a risk of sending line power to the patient and a risk of sending high voltage to the power cord during delivery of the defibrillation shock. Accordingly, a line-powered defibrillator usually includes electrical isolation circuitry to prevent the line power from reaching the patient and to prevent the defibrillation energy from passing through the power cord. Some isolation circuitry, such as a transformer, is large and heavy, adversely affecting the portability of the external defibrillator. In addition, a line-powered defibrillator cannot draw energy from an electrical outlet when the patient is far from an electrical outlet.
Other external defibrillators use one or more batteries as a power source. A battery-powered defibrillator does not require a power cord and therefore does not require the bulky isolation circuitry needed for a power cord. In conventional external defibrillators, batteries may be permanently mounted or replaceable.
When a defibrillator includes batteries that are permanently mounted in the device, the batteries usually are rechargeable. Many rechargeable batteries, such as nickel-cadmium batteries, sealed lead acid batteries or nickel-metal-hydride batteries, require a special recharging apparatus. The addition of recharging apparatus to the defibrillator adds to the bulk, weight and cost of the device. In addition, the recharging apparatus may be line-powered, which in turn necessitates electrical isolation circuitry for safety.
A further drawback to rechargeable batteries is short shelf life. Nickel-metal-hydride batteries, for example, discharge within a few months, even when no load is applied. Some rechargeable batteries, such as nickel-cadmium batteries, need to undergo conditioning cycles periodically to deliver optimum performance.
External defibrillators may include capacitors to smooth the power supplied by the rechargeable batteries. Although capacitors can be used as a rechargeable source of dc power, banks of conventional capacitors are incapable of supplanting the rechargeable batteries as a power source. Banks of conventional capacitors store too little energy, or are too bulky for a portable external defibrillator, or both.
As an alternative to permanent rechargeable batteries, a defibrillator may use replaceable batteries as a power source. Many replaceable batteries are not capable of delivering the energy demanded by the charging circuit. Replaceable batteries that can deliver the energy typically require heavy duty, low impedance connectors to carry the high currents safely, and the bulky connectors add to the weight and cost of the device. Heavy duty, low impedance conductors also increase the risk of inadvertent shock to an operator, because of the comparatively large size of the conductors.
In addition, some replaceable high capacity batteries pose additional hazards. When lithium sulfur dioxide batteries fail, for example, the batteries vent noxious gases, and when is lithium manganese batteries fail, the batteries vent flammable electrolytes.